Devices for pumping molten metal (referred to herein as pumps, molten metal pumps or molten metal pumping devices), particularly molten aluminum, and various components that can be used with these devices are generally disclosed in U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat. No. 5,203,681 to Cooper entitled "Submersible Molten Metal Pump," pending U.S. application Ser. No. 08/759,780, filed Dec. 13, 1996, entitled Molten Metal Pump With A Flexible Coupling and Cement-Free Metal-Transfer Conduit Connection, U.S. application Ser. No. 08/889,882, filed Jul. 3, 1997, entitled Rotor Bearing System for Molten Metal Pumps, U.S. application Ser. No. 09/152,168, filed Sep. 11, 1998, entitled Improved Gas Dispersion Device, U.S. Pat. No. 5,678,807 to Cooper and U.S. Pat. No. 5,662,725 to Cooper, the disclosures of which are incorporated herein by reference.
Three basic types of pumps are utilized: circulation pumps, transfer pumps and pumps including gas-release (sometimes referred to as gas-injection) devices. Circulation pumps are used to circulate the molten metal within a bath, thereby equalizing the temperature of the molten metal and creating a uniformly consistent alloy. Most often, as is known by those skilled in the art, circulation pumps are used in conjunction with a reverbatory furnace having an external well. The well is usually an extension of the charging well where scrap metal is charged (i.e., added).
Transfer pumps are generally used to transfer molten metal from the external well of the furnace to a different location such as a ladle or another furnace.
Pumps including gas-release devices circulate the molten metal while adding a gas into the molten metal stream created by the pump in order to "demag" or "degas" the molten metal. In the purification of molten metals, particularly aluminum, it is frequently desired to remove dissolved gases such as hydrogen, or dissolved metals such as magnesium. As is known by those skilled in the art, the removing of dissolved gas is known as "degassing" while the removal of magnesium is known as "demagging."
Known molten-metal pumps include a pump base that comprises a housing, also called a casing, a pump chamber, which is an open area (or cavity) formed within the housing, and a discharge, which is a channel formed within the housing that communicates with the chamber and leads from the chamber to an outlet formed in an exterior wall of the housing. A rotor, also called an impeller, is mounted in the pump chamber and connected to a drive system, which is typically one or more vertical shafts that eventually connect to a motor. As the drive system turns the rotor, the rotor creates a stream of molten metal by pushing molten metal out of the pump chamber, through the discharge, out of the outlet and into the molten metal bath.
When pumping molten metal it is desirable to increase the quantity of metal moved (or pumped) for each revolution of the rotor. Further, it is most desirable to increase the quantity of metal pumped per revolution without increasing the pump's size or energy consumption (or increasing the size and energy consumption by the minimum amount possible). Increasing the amount of molten metal pumped is desirable in any molten metal pumping device because it provides at least the following benefits: (1) in a circulation pump the temperature of the bath is more uniform; (2) in a gas-release pump more molten metal is demagged and/or degassed within a given time frame; and (3) in a transfer pump greater pressure is generated to push the molten metal upward to a higher elevation, if so desired. Alternatively, denser metals, such as zinc, can be pumped through the same transfer pump, having relatively standard pump components and relatively low energy consumption, as is used to pump molten aluminum.
It is also desirable to move more molten metal for each pump revolution because the rotor and pump can operate at a slower speed (i.e., fewer rotor revolutions per minute). This decreases vibration and hence the wear on the pump components, which leads to longer component life and lower maintenance costs.
Several rotors for molten metal pumps are disclosed in the prior art. All are deficient, however, in that they do not solve the above problems in an optimal manner.